According to [this study at MIT][1], the minimum number of cells that need to be populated to generate a solvable Sudoku is 17. If we ignore the definition of Sudoku (which states that there must be precisely one solution), then any 9x9 grid with 16 or fewer numbers on it will have multiple solutions.
[1]: https://www.technologyreview.com/s/426554/mathematicians-solve-minimum-sudoku-problem/